Convergent photon and electron beam generator device

ABSTRACT

A piece of scientific/technological equipment is presented for the generation of a convergent photon beam for radiotherapy or other applications. This equipment consists of adequately modifying the trajectory of an electron beam from a linear accelerator (LINAC) by applying magnetic and/or electric fields. These electrons perpendicularly impact the surface of a curved material that has a particular curvature ratio (anode), thus generating X-rays. The interaction of the electrons with the atoms of the anode&#39;s material generate X-rays with a non-isotropic angular-spatial distribution, with a greater concentration in the focal direction, which is defined by the geometry of the anode. A curved collimator with an adequate curvature ratio is attached to the back of the anode. The collimator is made up of an array of a great number of small holes that point toward the focal point. This device transmits X-rays solely in the focal direction. The Summary Figure presents a typical configuration of the invention that has been presented.

From the discovery of the X-ray in 1895 until now, the emission of aradiation ray, at any energy range, is essentially divergent and theintensity is a function of the distance between it and the emissionsource (inverse square law). This is due to the X-ray productionmechanism; in other words, electronsthat impact a target. There arecurrently several ways to generate X-ray beams, each with a determinedsource size and a specific, always-positive divergence. The X-ray beamsemployed in radiotherapy are divergent.

The expected objective of radiotherapy is, by using X-rays, to achieve ahigh X-ray flow zone within a specific volume. These X-rays would thendeposit their energy. The energy deposited per unit mass is known asdose in radiotherapy. Since the beam that is used is noticeablydivergent, several beams (fields) aimed at the volume of interest mustbe employed. As is widely known, the depth dose for an X-ray beam isdependent on an exponential downward curve according to the depth, witha maximum value near the surface. A multi-field application allows for amaximum dose in the interest volume (tumor site), despite the fact thatthe dose values in the surrounding areas are lower than those at thetumor site. These dose values are significant as they have higher valuesthan what is tolerable in some cases, which can prevent the use of aneffective dose in the tumor.

More refined techniques such as Intensity Modulated Radio Therapy (IMRT)or arc therapy, improve and conform the maximum flow volume of X-rays,thus lowering dose levels in neighboring tissues and organs, though thisdecrease is not significant. A dose value decrease of up to 80% intissues and organs near the interest zone has currently been achieved inrelation to the dose in the interest zone. Treatment planning, however,continues to be complex. A decrease of collateral effects caused byradiation is always attempted, though their complete eliminationisimpossible.

A radiotherapy technique that has lower collateral effects and greaterradiobiological effectiveness at the tumor site is that ofhadrontherapy. This technique uses hadrons (protons or heavier nuclei)to deposit high doses at the tumor site that are very conformed, that isto say, limited to that area. The cost of this technique, however, ismuch higher than conventional photon or electron methods, precluding itsuse for many patients. It is also rarely available at hospitals andhealth and treatment centers. FIG. 1 shows a graph comparing therelative depth dose of the most widely used radiotherapy techniques.

This invention proposes the use of a device able to generate aconvergent photon beam with advantages that are significantly greaterthan the conventional external radiotherapy technique and thehadrontherapy techniques, the latter catalogued as being those thatprovide better results.

From a comparative point of view, conventional conformal radiotherapytechniques or IMRT (the latter being better): administer a greatersuperficial dose; are a greater risk to healthy organs; requirefractioning and a more complex planning system; require more energy and,therefore, more costly bunkers; not all tumors are accessible; thusrendering the techniques less effective. The advantages of thesetechniques are that a greater volume is treated and the positioningsystem is simpler. FIG. 2 shows the fundamental difference between theconventional method, (a), and the convergent method, (b).

The convergent method, however, presents: lower surface dose, low dosein healthy organs; high dose in the tumor which does not requirefractioning; simpler planning system; shorter treatment (one or twosessions); greater effectiveness and accessibility to most tumors;simpler refrigeration system; high energy is not required thus bunkershielding requirements are lower. The disadvantage is that, as thetreated volume is smaller, a tumor scan and a more precise positioningsystem are required.

The only external photon method that is comparable, quality-wise, to theconvergent technique of the invention being proposed is the arc therapytechnique, also known as Tomotherapy, using photons with alinearaccelerator (LINAC). Arc therapy emulates convergence by using anangular scan around the isocenter (tumor site). Despite longer sessionsand equally complex planning, however, each beam is still essentiallydivergent and the doses in healthy organs are not insignificant. Likethe other conventional LINAC techniques, several sessions are required.Similar results can be obtained using a robotic device called a“cyberknife”.

The hadrontherapy technique presents the following: a low surface doseand is highly effective as it deposits a high dose depth in a very smallsite (Bragg peak, see FIG. 1). Hadrons and ions have highradiobiological effectiveness (protons are 5 times more effective thanphotons) and complex positioning systems. However, a very complexinstallation is required, which includes a synchrotron able toaccelerate particles to energies ranging from several hundred MeV toseveral GeV, high vacuum, and electrical and magnetic guide systems.Furthermore, the cost of a hadrontherapy system exceeds $100M USD. Thereare 28 hadrontherapy installations in the world's most developed nationsand the technique continues to grow despite its high cost. Hadrontherapyis out of the question for Chile at present though Spain is evaluatingthe possibility of acquiring one of these installations in the next fewyears. Hadrontherapy has shown excellent results in patients withcomplex cancers as it is able to treat tumors that cannot be treatedwith photons. The cost of this therapy, however, means it is availableto only a select few.

The convergent method employed by the invention presented here deliverslow surface dose and is highly effective, as it deposits a high depthdose in a very small area (“peak focus”). Photons have lessradiobiological effectiveness, but the dose deposited at the focus peaksite can be up to 100 times greater than the dose on the surface,despite the attenuation effect. This compensates for the photons' lowerradiobiological effectiveness and generates an even lower relative doseon the surface and in the healthy organs than that which is obtained inhadrontherapy. The positioning system, however, must be more precisethan that of conventional techniques. All of the above will allow forthe treatment of complex cancer cases as with hadrontherapy but with aless complex installation.

Furthermore, the cost of a LINAC plus a bunker and control building isin the $2 to 3 MUSD ranges, while a LINAC-adaptable convergence systemmay cost $0.5 M USD or less, a noteworthy advantage in relation to thecost of a hadrontherapy installation which is almost two orders ofmagnitude greater. In this regard, a convergent system would functionsimilarly to a hadrontherapy system but at a significantly lower cost.

The first step taken prior to the development of this invention was thestudy of the effects of a photon beam's convergence on a specificmaterial that was carried out by Monte Carlo Simulations (MCS) andtheoretical calculations. FIG. 3 shows the curves of a depth dosecorresponding to MCS and the theoretical results.

Devices currently exist that achieve beam convergence with a divergentX-ray beam based on the total reflection principle. The divergent X-raysenter a cone-shaped capillary, and the beams travel the length of it bytotal reflection inside the capillary until they reach the end. The exitsection is smaller than the entry section, thus allowing a greaterintensity to be achieved. In order to attain a significant increase inintensity, a set of these cone-shaped capillaries set in parallels isused. This makes up what is known as a poli-capillary and allows theentry area to be increased. However, as these devices employ the totalreflection principle, its use is only advantageous with X-rays withenergies below 50 keV, which limits its application in radiotherapyequipment, where the X-ray energy is much greater than theaforementioned amount. There is currently a great variety of X-rayfocusing devices that use not only the total reflection principle butdiffraction and/or refraction as well, though all of them can be usedfor low energy X-rays (<50 keV). For example, in astronomy, an X-raytelescope (Chandra and equivalents) obtains X-ray images of theUniverse, allowing us to see emission sources, including black holes.This is a large-scale device (several meters) that is based on the sametotal reflection principle and uses reflector plates and othermaterials.

After considering existing devices, which are limited to low energy, andthe results obtained from studies that were performed, this innovativeidea of an electron- and convergent X-ray-generating piece of equipmentwas developed, appropriate for low, medium and high energies (<0.1 MeV,0.1-1.0 MeV and 1>MeV respectively). This would also be the only way toachieve X-ray beam convergence at energies within the application'srange in radiotherapy techniques.

When this beam is pointed virtually towards a water phantom or waterequivalent, a depth dose profile can be obtained like the one shown inFIG. 4 for two different energies. These profiles were achieved using aMCS code. Other results attained by MCS are shown from FIGS. 5 to 8. Allthe MCS that were carried out show that the RTC convergent radiotherapytechnique, as proposed with this invention, is noticeably better thanthe conventional techniques used to date.

A very brief description of the positioning system for the various casesis given in this presentation of the invention. Directional arrows arealso shown without providing further details, as that would not be partof the essence of this invention. Also, positioning systems are alreadyavailable on the market. However, the various ways in which theinvention must be adapted in each case shall be presented.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a relative depth dose for the different techniques used inradiotherapy.

FIG. 2 a shows a diagram of the traditional X-ray radiotherapy system.

FIG. 2 b shows a diagram of a convergent X-ray radiotherapy system.

FIG. 3 shows a depth dose for a convergent photon beam at 0.4 MeV in awater phantom, compared to the theoretical results and MCS.

FIG. 4 shows a comparison of two dose profiles for convergent photonsfrom two MCS for focal points at 2 and 10 cm from the surface of thephantom.

FIG. 5 shows a sectional view of a depth dose achieved by MCS forconvergent photons, for a non-refined case.

FIG. 6 shows a sectional view of a depth dose achieved by MCS forconvergent photons, generated by the electrons that incite on an anodiccap and then pass through a perforated cap-style collimator similar tothose proposed in this invention.

FIG. 7 shows a profile of the energy deposited at low energy (per voxelunit) (Z=0, Y=0) with angular collimator acceptance: polar: 2 degreesand azimuthal: 2 degrees. E=4 MeV.

FIG. 8 shows a profile of the energy deposited at high energy (per voxelunit) (Z=0, Y=0) with angular collimator acceptance: polar: 2 degreesand azimuthal: 2 degrees. E=4 MeV.

FIG. 9 shows a graph of the convergent electron beam generator element,which can be made up of more than one magnetic lens.

FIG. 10 shows a graph of an alternate configuration of the convergentelectron beam generator element, which can be made up of anelectrostatic element similar to cylinder lenses.

FIG. 11 shows a graph describing how the convergent photon beam isgenerated.

FIG. 12 shows a graph of the present invention's essential parts.

FIG. 13 shows a graph of the whole invention, including each of itsparts.

FIGS. 14 a and 14 b show a sectional view and front view, respectively,of a front-end unit.

FIGS. 15 a and 15 b show a sectional view and front view, respectively,of an alternative configuration of a front-end unit.

FIG. 16 shows a configuration of the present invention adapted to atypical LINAC.

FIG. 17 shows a configuration of the present invention adapted to acyberknife.

FIG. 18 shows a configuration of the present invention adapted to aTomotherapy system.

FIG. 19 shows an alternate configuration of the present invention beingused for low energy applications.

FIG. 20 shows an alternate configuration wherein a flat collimator witha parallel beam exit is used.

FIG. 21 shows an exchange of a photon exit cone for an electron exitcone.

DETAILED DESCRIPTION OF THE INVENTION

The invention presented here consists of a device that generates aconvergent electron and X-ray beam. First, an electron beam from anelectron cannon is needed. The electrons are accelerated in aradiotherapy LINAC by a series of stages until a flow of electrons withenergy between 6 and 18 MeV is achieved. It can also be used forintermediate energies, known as orthovoltage energy (hundreds of keV),generated solely by an electron cannon.

As seen in FIG. 12, the relatively collimated electron beam coming froma LINAC is first expanded by an electron disperser. The electrons arethen focalized by the action of an appropriate set magnetic orelectrostaticlens (2). The electrons that emerge from the lens interceptthe surface of ananode shaped as a spherical (or aspheric, parabolic)cap (3), which shall be referred to as an “anode cap”. The anode cap'scurvature radius defines the focal distance (spherical) of theconvergent system.

As shown in FIG. 9, the magnetic lens has an entrance lens body (c), afield concentrator housing (d) and electric conductors with a solenoidwinding (e).

The convergent electron beam generator element, in an alternativeconfiguration of the invention, can also be made up of an electrostaticelement that is similar to cylinder lenses, which is in turn made up ofthree cylinders. The first is grounded (f), the second cylinder isnegatively polarized (g), and the third is also grounded (h). (See FIG.10).

The electronic lenses must be adjusted so that the electron beam impactsperpendicularly on the entire surface of the anode. As a result of theinteraction of the electrons with the atoms that make up the anode'smaterial, breaking radiation (known as bremsstrahlung), or X-rays in thematerial, is generated. As the incidence of the electrons occurs on theentire surface of the anode cap (i) (see FIG. 11), the bremsstrahlungX-ray emission phenomenon will occur isotropically on the entire cap(3). Bremsstrahlung is generated at each point on the cap. According toFIG. 11, the X-rays that exit the cap have an angular non-isotropicdistribution, with a greater intensity in the electrons' incidencedirection and an angular divergence inversely proportional to theincident electron's energy (k). The X-rays are then collimated by aspherical poli-collimator (5) (similar to the anode cap) with tens,hundreds or thousands of small holes (millimeter- orsub-millimeter-sized) pointing in the direction of the focal point (l).The X-rays that are able to pass through these holes will exit with amuch lower angular dispersion than they had at the anode cap exit (3).The rest are absorbed into the material, thus generating a convergentphoton beam, with its greatest intensity concentrated at the focalpoint. The definition of the focal point of this convergent photon beammay be improved by inserting a second poli-collimator cap (7). Thiseffect globally generates a radiation volume that mainly points towardsthe system's focal point with a significantly greater intensity ofX-rays at the focal point(peak-focus), the magnitude of which willdepend upon the energy of the electrons, the curvature radius of theanode cap (3), the anode cap's surface and the opening of the field'sdiaphragm that will be shown further on.

The invention's essential parts are shown in FIG. 12. Electrons comingfrom a source, whether a LINAC or an electron cannon, are dispersed by asmall sheet (scattering foil) (1) in order to generate a flow ofdivergent electrons. The electrons are diverted towards the axis by amagnetic (or electrostatic) lens (2), thus generating a flow ofconvergent electrons (i) that is perpendicularly intercepted a) by athin, cap-shaped (anode-cap), spherical, aspherical or parabolic anode(3), and a lateral beam breaker (4). The X-rays that are able to exitthe thickness of the anode (k) are collimated by a collimator cap (5)that has small holes perforated on its entire surface (6) that point inthe direction of the focal point. The convergent X-ray beam (l) can becollimated once again (m) by a second smaller poli-collimator (7) thatis similar to the first. This collimator is surrounded by a concentricring with a cone-shaped interior (8), which allows it to absorb the outof focus X-rays and decrease the beam's lateral penumbra.

FIG. 13 shows the invention as an apparatus in detail. It has anelectron source coupler (9), which allows the device to be attached to aspecific LINAC or a particular electron cannon. Whichever the case, itis a piece that must adapt to the different devices available on themarket or one that can be built solely for the convergent device. Above,in the middle section, there is a hole or window that allows the entryof electrons (10). Electrons coming from a LINAC can impact upon thepreviously described disperser (1), though the scattering foil is notnecessary as the electrons emitted by the cannon have an angularaperture. The electron beam enters a vacuum space contained by acone-shaped shield (11), with a vacuum connection (12) and at the baseof the cone there is a ring-shaped support (13) that attaches to theexternal cylindrical housing (14). Further down is a phase coupler (15)that separates the electron part from the photon part. The device'sphoton part is made up of an external housing shaped like a truncatedcone (16) that has internal shielding (17) with supports for pieces (4)and (8) as well as a vacuum connection (18) if required. Finally, thereis a frontal unit (19) at the exit of the convergent beam, at theinferior end of the truncated cone. This frontal unit has positionsensors, location laser lights and a mechanism that regulates fieldsize, which in turn regulates the intensity at the focus peak. Theunit's details and versions are described below.

FIGS. 14 a and 14 b show two views of the frontal unit, which is made upof several diaphragms (20), placed one on top of the other, thatregulate the size of the exit radiation field. In order to mark theentry field on the surface of the patient, there is a frontcover (21)made out of a low Z (atomic number) material, such as acrylic, withholes in which small laser guides or laser diodes (22) are placed thatpoint in the focal point's direction. These are located along acircumference on the border of the field diaphragm, enabling visibilityof the entry field upon the patient's skin undergoing treatment withthis device. Given that the machine-patient positioning in thisconvergent radiotherapy technique can become a critical factor, thedevice has sub-millimetric precision sensors and/or reflectorson thefront (23). Finally, in order to locate the axis of the isocenter focuscone, the apparatus has a small removable central laser guide (24).

FIGS. 15 a and 15 b show two views of an alternate frontal unit in whichtwo diaphragms are replaced by a solid interchangeable conical ring (25)that has a predefined field size. The surrounding laser guides can beincorporated into the front of the ring and the position sensors andcentral laser guide may be located on the acrylic or equivalent (low Z)cover, similar to that found in the previous figure.

FIGS. 16 and 18 illustrate how this invention could be adapted toapparatuses currently in use for external photon radiotherapy. FIG. 16exhibits the invention adapted to a LINAC, showing an accelerator (26),the deflector magnet (27) and the invention being proposed here (28).The figure also demonstrates how the gantry would be replaced with theconvergence device. Positioning system, table scan and head rotationwith high precision position sensors on the contour of the fieldaperture diaphragm of the device and on the patient's skin.

FIG. 17 shows a configuration of the invention adapted to a cyberknife:robotic system (29), small linear accelerator (30) and the invention(28). It already has a positioning system, movement and high precisionsensors.

FIG. 18 displays a configuration of the invention adapted to aTomotherapy device. Rotation system (31), small linear accelerator (30)and the invention (28). Positioning system and gurney scan (x) and headrotation with radial movement of the apparatus (original part of theapparatus). Position sensors are added to the contour of the fieldaperture diaphragm of the device and others are adhered to the patient'sskin. This generates feedback signals so that the positioning and scansystem is more precise.

The above means that the device is built a certain size so that it isadjustable to the size of the device to which it is adapted, providedthat its entry diameter is the same size as the exit diameter of thedevice to which it will be adapted (gantry or beam exit cannon).

FIG. 19 shows a prototype for intermediate energies in the ortho-voltagerange. This prototype is made up of an electron cannon (32) for energylevels of several hundred keV and the convergent X-ray device beingproposed (28). The electron cannon is comprised of a filament (33), aconcentrator cathode (34) and an accelerator anode and disperser (35).It is also equipped with sensor systems for feedback positioning usingthe devices described above as well as sensors adhered to the patient'sskin (similar to a bandage) (36). Additional advantages that a unit suchas this one has to offer are its noteworthy low cost, small size andfewer shield requirements, thus making external photon radiotherapy aneffective, low-cost technique available to a greater number of people.

Another application is proposed in FIG. 20 as an alternative to theinvention presented here. It would be applied in the specific case thatthe anode and collimators' curvature radius were to mathematically tendto infinity while maintaining the normal incidence condition of theelectrons on the anode. In other words, by placing a flat collimator(37) like the one shown, the X-ray beam will be parallel and homogeneousupon exit. However, this is destined for other applications wherein theconvergence of the beam is not required, as in images.

Lastly, the description in FIG. 21 explains how the proposed convergentphoton beam unit can be converted to a unit with a convergent electronbeam exit by exchanging the photon exit cone for an electron exit cone(38), as shown.

1. A convergent X-ray beam-generating device, comprising: an electrondisperser element and one or more magnetic and/or electric lenses thatcan expand and redirect the electron beam coming from a source in such away that the emergent beam impacts perpendicularly upon the surface ofan anode cap made of a material and thickness such that bremsstrahlungradiation is generated on the entire surface that is impacted by theelectrons; part of the X generated radiation is emitted forward, aimingmainly at the anode cap focal point, and the radiation is thencollimated by a coaxial poli-collimator in the same shape of the anodecap, with multiple holes aiming at the focal point.
 2. A deviceaccording to claim 1 wherein an electron beam can be provided by ahigh-energy linear LINAC accelerator or an electron cannon (morecommonly known as an electron-gun) for intermediate and low energy.
 3. Adevice according to claim 1 wherein electron lens system is made up ofcontrollable magnetic and/or electric fields and by magnetic and/orcondenser bobbins.
 4. A device according to claim 1 wherein the anodecap can be spherical, aspherical, parabolic o another shape with ageometry that has a focal point.
 5. A device according to claim 1wherein the poli-collimator has tens, hundreds or thousands of holesaiming in the direction of the focal point.
 6. A device according toclaims 1 wherein the poli-collimator can be either adhered to theconcave part of the anode cap or separate.
 7. A device according toclaim 1 wherein the material from which the collimator is made is of acertain composition and thickness, such that it is able to completelyattenuate the X-rays that impact outside the collimator's holes and theholes can be either cylindrical or conical.
 8. A device according toclaim 1 wherein the collimator has a pattern of holes of specificmeasurements which can be of circular, square, hexagonal or randomsymmetry.
 9. A device according to claims 1 wherein the beam emergingfrom the collimator can be re-collimated by a second similar and smallercone-shaped collimator that has the same hole pattern as the first, andis coaxially located in front of the second.
 10. A device according toclaim 1 wherein the penumbra and out-of-focus beams are externallyremoved from the cone-shaped beam emerging from the firstpoli-collimator and are removed using a cone-shaped ring that coaxiallysurrounds the second poli-collimator or is coaxially aligned in front ofit.
 11. A device according to claim 1 comprising a frontal unit with oneor more diaphragm-type devices, similar to those used in optics (visiblelight range) that regulate the size of the exit radiation field.
 12. Adevice according to claim 1 wherein a removable disk made of low Zmaterial (acrylic or similar) is located on the front of the frontalunit, upon which small laser guide diodes and position sensors areaffixed.
 13. A device according to claim 1 which adapts to aconventional LINAC as it can have measurements and an attachment piecesimilar to that of the LINAC head, enabling it to be attached there inplace of the conventional head in such a way that the ensemble canbecome a convergent beam device.
 14. A device according to claim 1 thatis of a smaller size and has an attachment piece that allows it to befixed onto the front part of a small LINAC, a cyberknife or aTomotherapy device, taking the place of the above units' traditionalX-ray piece; thus the entire ensemble becomes a convergent beam device.15. A device according to claim 1 that is of a smaller size, an electroncannon can be attached to it so that the ensemble becomes a stand-aloneconvergent beam device.
 16. A device according to claim 1 wherein thefrontal unit's position sensors in each adaptation of this device can beattached to small position sensors or their equivalent, located on thepatient's skin and the signals that are generated can be used to controlthe step-step motors of tables, gantries, robotic arms or other means ofpositioning, compensation or scanning.
 17. A device according to claim 1wherein, based on the operating principle, the device can be convertedso as to obtain a flat parallel photon beam, which can be achieved byswitching the anode cap for a flat sheet and replacing the cone-shapedcollimators with flat collimators with their respective accessories,adjusting the electron lenses so as to keep the electrons' incidenceperpendicular upon the anode flat sheet.
 18. A device according to claim1 wherein the front cone-shaped piece that generates convergent photonsmust simply be removed in order to obtain a convergent electron beam.19. A device according to claim 1 wherein the front photon cone can bereplaced by another similar, empty electron cone, on the exit of whichan equivalent front unit may be attached and must have the same lateralguide lights and position systems and position sensors previouslydescribed in the claim.